This invention relates generally to crystal pulling machines and more particularly to a method and apparatus for rotating a crucible of a crystal pulling machine.
The substantial majority of monocrystalline silicon used to make silicon wafers for the microelectronics industry is produced by crystal pulling machines employing the Czochralski method ("CZ method"). In brief, the CZ method uses a silicon melt formed by melting chunks of high-purity polycrystalline silicon in a quartz crucible in a furnace. A seed crystal is mounted from a crystal pulling mechanism above the silicon melt. The seed crystal is lowered by the crystal pulling mechanism into contact with the silicon melt. After the end of the seed is melted, the silicon melt is cooled until the crystal begins to grow. The crucible is rotated in one direction and the crystal is rotated in the opposite direction as the crystal is pulled from the melt. As the crystal grows, it solidifies silicon from the silicon melt in the crucible.
During the growing process, an oxygen diffusion boundary layer forms in the silicon melt near the crucible wall. This causes a certain level of oxygen to be present in the pulled crystal. When conventional crystal pulling techniques are used, an oxygen gradient forms radially in the crystal as well as along the length of the crystal.
In order to lessen the oxygen gradient in a grown crystal, the prior art teaches rotating the crucible in an on/off mode or with a pulsed square wave superimposed over a constant speed of rotation for the crucible. An example of the pulsed wave method is shown in U.S. Pat. No. 5,215,620 which superimposes a pulsed square wave over a constant reference speed of rotation for the crucible during the entire crystal pulling process. Although this increases the oxygen concentration in the back end of the silicon crystal rod, it also increases the oxygen concentration in the front end where increased oxygen only contributes to the axial oxygen gradient. Although these prior art methods reduce the oxygen gradient somewhat in a grown crystal, there is room for improvement. Further, these prior art methods cause unnecessary mechanical shock on the motor and drive train which rotate the crucible. Such mechanical shock causes a loss of energy in the motor and drive train and increases the wear rate for these components.